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Akai GX-620 Reel to Reel Tape Deck Measurements

MRC01

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Frequency response:
...
Yeah, this is pretty ugly compared to what we're used to seeing on this website. As I have always thought, there is elevation in low frequencies. There is also a bulge around 50hz which I'm guessing it due to mains in the UK. I wonder if this would show any improvement with re-capping the power supply board? I have only re-capped the pre-amp board so far.
...
Looks to me like you need to fine-tune the bias to match the tape. A bit less would tilt the FR, lowering LF and raising HF. That's what I got testing a Denon DRM-540
PS: this doesn't mean the bias is bad or faulty, just not optimized for this particular tape. Every tape requires a slightly different bias to achieve flat response, which is why many decks have a bias fine adjust knob on the front panel.
 

30 Ounce

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Bias is almost always user adjustable. On professional recorders like Revox it was standard operating procedure to set bias.
 

dfuller

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Tape speed = 7.5ips.
This explains a lot of the frequency response - lower tape speeds have more lumpy frequency response. Fidelity gets "good" at 15ips. Bias definitely plays in as well...
 

gvl

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I’ve been scarred not being able to afford one of these Akai R2R decks with GX heads back in my teenage years. Thinking about it, those scars still hurt :)
 

mhardy6647

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I am trying to locate a review index covering the japanese Stereo Sound magazine. Any ideas?
no, sorry :(
Maybe reach out to Joe Roberts (who, last I knew, is still associated with Silbatone and who'll probably be at the Capital AudioFest in Bethesda, MD next month)? He might know of one -- were there one. :) I presume you mean in English(?).

 
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MRC01

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This explains a lot of the frequency response - lower tape speeds have more lumpy frequency response. Fidelity gets "good" at 15ips. Bias definitely plays in as well...
7.5 ips should be fast enough for flat FR when the deck is calibrated. I've measured cassette decks that are +/- 2 dB from 20 Hz to 20 kHz, running at 1 7/8 ips, or 1/4 that speed.
 

mhardy6647

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This explains a lot of the frequency response - lower tape speeds have more lumpy frequency response. Fidelity gets "good" at 15ips. Bias definitely plays in as well...
Well, yes and no. I believe the head bumps in the LF start to rear their (ahem) heads at 15 relative to 7.5 -- but of course, the LF performance gets worse at higher tape speeds. :confused:

rando google illustration thereof ;)
tapelo3g.JPG

source: https://www.tangible-technology.com/media/media_2.html
Looks like some bias issues (maybe) goin' on as well... ?
 

mhardy6647

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I’ve been scarred not being able to afford one of these Akai R2R decks with GX heads back in my teenage years. Thinking about it, those scars still hurt :)
Hail -- that's why I seem to be building up a collection of 'em now. :)
Not AKAIs, though.

PS Still cannot afford a really nice one... but I do know some guys...

:cool:
 
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eliash

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I recently had the idea to use my hi-spec ADC in combination with a USB sound card to have a go at creating my own measurements. This is the first time I've done this, so keep in mind there may be some mistakes etc. This is not my area of expertise.

View attachment 239012

Above is my beautiful Akai GX-620.

I replaced all the dodgy Akai transistors and swapped out the electrolytic capacitors whilst I was at it. I have always been quite surprised by how good this thing sounds when listening to the source. However, I do find there is a bit too much bass when recording to tape. This could be caused by many things. I don't imagine it is particularly well calibrated, and the tape heads have probably seen better days. However, I've wanted to have a go at testing and measuring something for a while now, and this felt like the perfect starting point.

Devices used for capture:
  • Desktop PC running Windows 11.
  • SMSL SU-8 v2 DAC.
  • Nihtila ADC AK5572
  • SoundBlaster SB1240
  • Multitone software.
Chain of devices (in loop): Windows PC -> SMSL DAC -> Akai GX-620 -> Nihtila ADC -> SoundBlaster 1240 (via Toslink) -> Windows PC

Documentation & Notes:

  • The SMSL DAC has already been reviewed and measured courtesy of our friend Amir: Click Here
  • Audio Precision results for the ADC can be found on Tomi Nihitila's website: Click Here
  • Toslink connects from the ADC to SoundBlaster 1240.
  • SoundBlaster Connects to PC via USB.
  • SoundBlaster responsible for Digital-to-Digital conversion only.
  • Bitrates all set to match between DAC and ADC etc (96khz).
  • Measurements have not been offset to 0db.
  • ADC is being used single-ended and not balanced which loses 6db of dynamic range.
The below measurements shows the SMSL DAC plugged directly into ADC:

1Khz Tone:

View attachment 239013

I was very surprised to see how similar it is to Amir's review. It is more or less identical. So nothing much to see here!

Moving on to a 32-tone signal:

View attachment 239014

Seems to be behaving as I would expect. Although, one thing to keep in mind, the above is not offset to 0db.

Frequency Response:

View attachment 239015

Nothing exciting to see here. Good and flat as you would expect.

Moving on to something more exciting... the below measurements show the tape deck hooked up between the DAC and the ADC.

First I measured the pre-amp alone (not recording).

1Khz Tone:

View attachment 239017

The above result was better than I was expecting! I reduced the voltage down slightly to not overload the pre-amp board, hence why the fundamental is 10db down. Also bit of mains noise going on (50hz here in the UK) and some harmonic distortion.

32-tone signal:

View attachment 239018

Doesn't seem too bad, noise floor would be much higher (almost 30db) if you were to offset the results for calculating SNR etc. Looks like the same mains noise going on.

Frequency response:

View attachment 239019

Seems flat enough to me. Little roll-off either end, but nothing major wrong here.

Now, the really interesting part! Let's record these tones to tape and see what happens. I used SM911 tape from Thomann.de

1Khz Tone:

View attachment 239020

Harmonic spikes have increased in size. Noise floor has also raised due to tape hiss. Personally, I am still pleasantly surprised by this result. I was expecting to see absolute carnage!

32-tone signal:

View attachment 239021

I'm afraid you guys are more or less on your own with the above result here. There is seemingly lots of distortion and the noise floor is uneven around 50-100hz. Also, you can see the frequency response is looking a little sloped down from the bass side of things. More below.

Frequency response:

View attachment 239022

Yeah, this is pretty ugly compared to what we're used to seeing on this website. As I have always thought, there is elevation in low frequencies. There is also a bulge around 50hz which I'm guessing it due to mains in the UK. I wonder if this would show any improvement with re-capping the power supply board? I have only re-capped the pre-amp board so far.

All in all, I was surprised by the above. I was expecting it to be a total horror show. Apart from the uneven frequency response, it doesn't seem too bad. However, as I first mentioned, I am not an expert in this area by any stretch of the imagination. I am merely a hobbyist.

I hope anyone who took the time to read this post has managed to get something out of it. Unfortunately, I won't be the best at explaining what it all means. However, I will say that, I was always surprised by how good this deck sounds. Obviously it doesn't sound like a modern day DAC, but I would never expect it too. I always had noticed the recordings from the deck had more bass, and sounded a little muddy when compared to the source. It's nice to see the data back this up by showing the uneven frequency response of the recording when compared to the pre-amp only measurement. Unsurprisingly, apart from the SU-8 alone, none of the above measurements look like what we're used to seeing here.

As a small side note. I can highly recommend the AK5572 ADC. What a fantastic little board that is. Coupled with the SPDIF converter here, the result is a truly fantastic performing ADC once boxed and fed with a decent PSU. I cannot remember exactly what PSU I used now, however it was a small silent-switcher style design. Clearly the above demonstrates that all working too!
Great, remember my adjustments to a Technics RS-1506 in the 80´s and 90´s,
- you should try to reduce the HF-bias to beef up high frequencies, then frequency response would look much better.
Anyway my RS-1506 still waiting in the basement for a complete recapping and mech. restoration...anybody interested?
 

mhardy6647

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Great, remember my adjustments to a Technics RS-1506 in the 80´s and 90´s,
- you should try to reduce the HF-bias to beef up high frequencies, then frequency response would look much better.
Anyway my RS-1506 still waiting in the basement for a complete recapping and mech. restoration...anybody interested?
As a fellow with a nice Pioneer RT-909 a-sittin' in the basement also waiting for mechanical rehab (i.e., by me)... umm, yeah, no. Sorry. ;)

More on topic: definitely thinking (i.e., agreeing) that tweaking the bias will help our OP, but his rolloff appeared smooth, and lowering bias tends to produce a hump in the high treble, doesn't it?
 

Cuniberti

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Without proper playback, recording, and head azimuth alignments, there is no way this tape recorded will perform anywhere near spec.
To do this would require experience, an MRL playback alignment tape, dual trace scope, a multi-tone generator, a manual, and the proper tools.
Maybe take it to a recording studio that still uses tape and find a tech who knows what they are doing, good luck.
 

bt3

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I bought a newly released GX 630D when I was serving military duty on border with East Germany back in Cold War years.
I liked that reel-to-reel, but not as much as my Revox. I know many young people today get kind of hypnotized watching
their vinyl records spinning 'round, however, watching those reels turn and UV meters glow is even more mesmerizing. (v:
 

MRC01

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... More on topic: definitely thinking (i.e., agreeing) that tweaking the bias will help our OP, but his rolloff appeared smooth, and lowering bias tends to produce a hump in the high treble, doesn't it?
Yes. Bias will help flatten it but it really needs a proper azimuth adjustment to get ideal flattest FR.
 

KaLam1ty

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These are absolutely beautiful machines.

I have the GX-625 model as well and it's a standout conversation piece (an eye distraction from the Genelecs too, thankfully lol). I don't have much "official" media to play on it, but they're fun and quite mesmerizing to play around with.. making instrument recordings, dubs, mix tapes, etc..

But based on its similarity with the 620, good to know it's not a completely toy either. The measurements seem comparable to the manual product specs, too.

PXL_20221024_200046335.jpg


The machine might be decade older than me; but I can't help but wish that today's equipment, be it professional or domestic, all looked like "this".
 

iraweiss

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Looks to me like you need to fine-tune the bias to match the tape. A bit less would tilt the FR, lowering LF and raising HF. That's what I got testing a Denon DRM-540
PS: this doesn't mean the bias is bad or faulty, just not optimized for this particular tape. Every tape requires a slightly different bias to achieve flat response, which is why many decks have a bias fine adjust knob on the front panel.
If you have not done so, download the service manual from hifiengine.com


You can also download the brochure from that page.

The service manual should have a bias procedure. As a start, there might be a front selector switch for bias and/or equalization that will get you in the ball park. Then you can bias the Akai to the tape of your choice. I use Maxell myself.

You may find that as you decrease the bias to raise the high frequencies the distortion and noise go up. You have to find a compromise with analog tape between frequency response, distortion and signal to noise ratio. There is no perfection in this arena.
 

JaccoW

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I bought a newly released GX 630D when I was serving military duty on border with East Germany back in Cold War years.
I liked that reel-to-reel, but not as much as my Revox. I know many young people today get kind of hypnotized watching
their vinyl records spinning 'round, however, watching those reels turn and UV meters glow is even more mesmerizing. (v:
That's why I do both. ;)

Recording records onto tape is a great way to make long continuous recordings of multiple records. Especially since the big ones can hold like 180 minutes on each side at 7½ ips.
 

Curvature

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Better than vinyl, cassettes, wire and wax. The cream of the audiophile analog crop.
 
OP
Stephen H

Stephen H

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FYI, I quickly tried the frequency response test again. The result is virtually identical no matter what level I record at. I am quite certain a full calibration could flatten it out a bit. If I ever get round to this I will be sure to put up some post-calibration results. However, I should probably prioritise re-capping the PSU board. I would hate for it to go bang or completely die on me. Also, given that I've already re-capped the pre-amp board, it makes sense to finish the job before any go bad or start leaking electrolytic fluid.
 
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